Insights into catalytic reduction of dyes catalyzed by nanocomposite beads Alginate@Fe3O4: Experimental and DFT study on the mechanism of reduction

In the present work, a facile pathway was developed to prepare magnetic alginate nanocomposite beads (M@AN). The prepared nanocomposite beads were evaluated as a catalyst for the degradation of methylene blue (MB) and orange G (OG) dyes in a simple and binary system under the presence of a reducing agent NaBH4. The in-situ formed magnetic nanoparticles (Fe3O4-NPs) were identified by visual observance, X-ray diffraction (XRD), UV-Vis DR Spectroscopy (UV-vis DR), Fourier transform infrared (FTIR) spectroscopy and energy dispersive x-ray (EDS). The obtained results revealed the formation of dispersed Fe3O4-NPs inside the beads linked with-COO groups of alginate biopolymer by a bidentate bridging coordination. Owing the presence and well -dispersed Fe3O4 nanoparticles in M@AN surface. The prepared catalyst was more efficient with the cationic MB dye in the simple and binary system. In the simple system, the rate constant was 0.0034 s(-1) and 7.416 10-4 s(-1) for MB and OG dye, respectively, whereas in the binary system the rate constant was 0.0012 s- 1 and 5.096 10-4 s(-1) for MB and OG dye, respectively. To have deep insight into the reduction mechanism of MB dye onto M@AN, the density functional theory (DFT) method was used. The results showed that the NaBH4 and MB re-agents are firstly diffused onto the surface of the nanocatalyst via hydrogen banding, then, the reduction reaction was carried out by a nucleophilic attack of borohydride ion by hydrogenation on the double bond between carbon and nitrogen in -N--C- group of MB which thereafter transformed into -NH-CH- bond. The theoretical results show good agreement with the experimental data, in which the calculated UV-visible spectrum of Leu-co-MB product using DFT method shows an electronic transition at 300 nm which is a close value to the experimental.